Dave Russel

I've been reviewing the dark matter and MOND literature quite extensively during the last year. The Clowe et al paper released earlier this week has raised quite a stir, but I believe the result is not as fatal to MOND, nor as much a "proof" of collisionless cold dark matter as is being claimed.

I have some doubt that the underlying premise of Clowe et al that the dominant mass in the cluster is necessarily in the form of X-ray emitting gas is correct. In following your papers I've noted that while the success of MOND is convincingly demonstrated for individual galaxies, the residual mass discrepancy in clusters is something that concerns you and now here with the 'bullet cluster' we have another galaxy cluster supposedly creating difficulties for MOND.

I have some speculations about the mass discrepancy in clusters that might lessen the difficulty for MOND, but if we assume the dynamical mass estimates for clusters and the recent 'bullet' results are accurate, an interesting paradox is raised. Clowe et al claim that their results prove that there is DM and that it must be collisionless - but what then are we to do about the results on galaxy scales - where in my opinion CDM is for all practical purposes falsified?

That list doesn't include the missing satellites and cuspy core problem of CDM simulations, but the eight items above are straightforward empirical results which demonstrate that on galaxy scales the distribution of the inferred DM is strongly coupled to the distribution of baryonic matter. This coupling is a property difficult ('impossible' in version 1 of the Donato& Salucci paper posted on ArXiv before the referee apparently got involved) to explain with a collisionless particle such as CDM.

My point - which I know is not news to you - is that this coupling between the luminous matter and the inferred DM underscores the failure of collisionless CDM on galaxy scales and remains a skeleton in the closet of Lambda CDM cosmology. But I wonder how many researchers of galaxy cluster scales and larger are aware of the severity of this skeleton? Do Clowe et al realize when they claim that their result proves there is collisionless CDM, that a much larger volume of research can be brought forward showing that on galaxy scales CDM is a complete failure???

It seems to me that there is a significant contradiction or paradox present in these results. If the Clowe et al results are accepted as correct, then it would appear there must be CDM, but galaxy scales say no - there isn't CDM. Which results should then be accepted? In the introduction of their paper released this week clowe et al state: 'Previous works aimed at distinguishing between the dark matter and alternative gravity hypotheses in galaxies (Buote et al 2002; Hoekstra et al 2004) or galaxy clusters (Gavazzi 2002; Pointecouteau & Silk 2005) have used objects in which the visible baryonic and hypothesized dark matter are spatially coincident, as in most of the Universe. These works favor the dark matter hypothesis'

There is a major and -whether intentional or not - severely misleading sin of omission in this 'review' of the DM literature. Perhaps the lensing studies they cited favor DM, but the lensing evidence is not all there is. The eight items I noted above are direct empirical evidence with fewer layers of assumptions.

In my opinion, the collisionless CDM supporters need to face this discrepancy in the evidence.
There is an interesting paradox at work here when on the scale of clusters and superclusters collisionless CDM succeeds while MOND struggles. Yet on galaxy scales the situation is reversed. This paradox should not be brushed aside by CDM researchers.

I also wonder if this assumption that the bulk of the mass should be the X-ray detected gas and should be offset in a MOND scenario is correct. The data could just as easily be used as a test of the gas temperature mass estimates. For example, given the weak lensing analysis, what mass would need to be present in the X-ray data to be consistent with TeVeS?

Given the almost complete failure of collisionless CDM on galaxy scales, I'm inclined to think that there are flaws in some of the many assumptions involved in galaxy cluster analyses.